Multifrequency receivers generally have an automatic switching system for selecting tuning elements to provide for signal reception on any one of a plurality of different channels. The channels can be selected automatically to selectively connect the different tuning elements or circuits in the receiver to each channel until a carrier wave or signal is detected on a channel. The receiver then terminates the channel scanning or selecting until the signal terminates, when the channel scanning will be resumed.
If often is desirable to assign a priority to one or more of the channels and to receive the signal on the highest priority channel whenever a signal is being received on the priority channel. One such system is disclosed in U.S. Pat. No. 3,614,621, which is assigned to the present assignee. In a typical priority channel system, the priority channel or channels are continually sampled for signals during the reception of signals on any of the other nonpriority or lower priority channels. Whenever a priority signal is detected, only the priority signal is received until the priority signal terminates.
In a priority signal system, the length of time that each channel is sampled when no signals are detected should be relatively short to permit the rapid scanning of all the channels for a received signal. If a signal is detected on a nonpriority channel, the system will remain tuned to the nonpriority channel for the major portion of time the signal is received and sample the priority channel or channels periodically. When a signal is detected during the sample period, the system will remain tuned to the priority channel until the signal terminates. When all signals terminate, the system will again scan all the channels until the next signal is detected.
The sample time or period is of sufficient length to detect the presence of a signal on the priority channel, but is short enough to prevent the production of a substantial audible hole in the signal being received on the nonpriority channel. The priority sample periods are generally taken at fixed intervals, such as four times a second. The sample periods are typically of 10 to 20 milliseconds in length. The sample periods do not cause a substantial audible signal hole in the nonpriority signal; however, the signal is muted during the sample period which can cause both annoyance to the listener and some loss of signal intelligibility.
It is therefore a general object of the present invention to provide an improved priority channel monitoring method and system which provides a minimal signal loss on the nonpriority channels.
It is a further object of the present invention to provide the minimal signal loss by sampling the priority channel during nulls in the received nonpriority detected audio signal.
It also is an object of the present invention to provide an enhanced priority channel sensitivity, by allowing longer sample length times without further degrading the nonpriority signal intelligibility.
It is another object of the present invention to set a minimum time period between priority sampling periods to avoid substantially continuous priority channel sampling.
It is an additional object of the present invention to set a maximum time period between priority sampling periods to avoid signal loss on the priority channel.